Recording devices, in which a heat-sensitive recording material having a heat-sensitive recording layer containing a leuco dye, a developer and a binder and formed on one side of a support composed of paper, synthetic paper, plastic film or the like is used as a recording media, are compact and inexpensive and are also easy to maintain. Therefore, in addition to being used as recording media in facsimile systems, automatic ticket vending-machines, scientific measuring instruments and so on, they are also extensively used as output media in printers or plotters for POS labels, CAD, CRT medical images and the like.
For printers for CRT medical measuring instruments that require uniformity and high resolution in the recorded images and for CAD plotters that require dimensional stability and fine-line recording, heat-sensitive recording materials employing synthetic papers having multi-layer structures and heat-sensitive recording material employing a biaxially oriented thermoplastic resin film optionally containing inorganic pigments are used.
Expansion of the field of use of such heat-sensitive recording materials has led to an increasing demand for a heat-sensitive recording material in which a recorded portion (i.e., recorded image), formed by a color-forming reaction between a leuco dye and a developer, is excellent in stability against chemicals such as plasticizers, edible oils, cosmetics and the like, and for a heat-sensitive recording material which gives a recorded portion exhibiting less change over time in density.
Heat-sensitive recording materials, in which at least two kinds of hydroxy-diphenylsulfone compounds such as 4-hydroxy-4′-isopropoxydiphenylsulfone, 4,4′-dihydroxydiphenylsulfone and the like are used as a developer to increase the stability of a recorded portion, are disclosed in Japanese Unexamined Patent Publications Nos. 1990-020385, 1990-249690, 1993-286255, 1995-172068, 2000-263944 and 2001-001647. However, there are further demands for less change in the density of a recorded portion over time caused by humidity and temperature, and for improved gradation of the recorded portion.
In some applications, heat-sensitive recording materials are required to give a recorded portion with an achromatic black color (or pale charcoal color, gray). Japanese Unexamined Patent Publications Nos. 1993-254254 and 1996-324130 disclose heat-sensitive recording materials in which a combination of a black-color-forming leuco dye and a red-color-forming leuco dye is used to give such an achromatic black color to a recorded portion thereof. However, in such heat-sensitive recording materials, further improvements are demanded in the recording sensitivity and the hue or gradation of a recorded portion thereof.
The term “gradation” employed herein means that the recording energy applied and the density (optical density) of the recorded portion (recorded image) are substantially directly proportional (linearly proportional). More specifically, a recorded image is formed with a density proportional to the intensity of recording energy applied; and the density of the recorded portion (recorded image) and the recording energy applied are in a nearly linear relationship. As a result, when multiple recorded portions (recorded images) are formed by changing (increasing or decreasing) the intensity of the recording energy applied in a stepwise manner, the resulting recorded portions (recorded images) exhibit clear gradation throughout their density range (from the lowest density to the highest density). Excellent gradation leads to excellent image quality.